A dental crown is one of the most important restorations in dentistry in that it affords the restoration of deteriorated teeth to a state of health and function. One of the prime requisites for a successful dental crown is that it adapt as perfectly as possible to the prepared tooth structure (i.e. the tooth is milled to a desired shape upon which the dental crown is placed). Anything less than substantially perfect fit, especially at the crown margin, can result in mechanical failure such as crown fracture or loosening or biologic failure such as decay, periodontal disease, or occlusal problems. At the present state of the art the materials available for the fabrication of crowns include the noble metals such as gold base alloys, acrylics such as polymethyl methacrylate, dental porcelain, or a composite of a metal and a compatible porcelain or acrylic.
At the present date the most widely employed method of fabricating dental crowns is to cast a metal substrate which is subsequently covered with a dental porcelain veneer.
The metal substrate is obtained by an indirect process known as the "lost wax" technique. Specifically, the substrate is first formed and shaped in wax. This involves making an impression of the prepared tooth, casting dental stone against the impression to form a master die, molding a body of wax against the master die and then shaping the upper portions of the wax body to duplicate the external shape of the tooth crown desired. The wax form must then be accurately converted to the metal. To accomplish this the wax form is invested into a refractory material, thus forming a mold. The wax is then eliminated by melting or burning, thus creating a cavity into which the metal, heated to a molten state, is cast. Hence, the wall of the cavity intended to be an exact replica of the prepared tooth can suffer inaccuracies because of the three-step technique needed to prepare it--preparation of the master die, preparation of the wax form from the master die, and preparation of the mold from the wax form. This renders the process technique-sensitive. The process is even further technique-sensitive in that each metal alloy possesses a specific casting shrinkage for which there must be accurate compensation. This compensation is critically dependent upon the proper selection and skillful manipulation of all materials employed. The technique is sensitive in that failure can occur at any of the numerous steps involved, such as distortion of the wax form, improper investing procedure, inadequate expansion of the mold, improper burn-out procedure, or improper melting and casting of the metal. A principal drawback of this technique is that many hours are consumed before a failure in the casting stage can be detected. The success or failure of the composite ceramic-metal crown is also greatly dependent on the proper finishing and handling of the surface of the metal to which the ceramic is applied. Proper finishing which is difficult and time consuming, is essential to the successful bonding of the ceramic to the metal substrate. Still further, the inherent physical differences between the porcelain veneer and the metal substrate, such as differences in thermal expansion, give rise to numerous potential avenues for failure.
An alternative technique, which eliminates the need for a metal casting, is to form the crown of a dental porcelain with a thin underlayer of platinum foil. The thin platinum foil is carefully shaped to a replica of the prepared tooth and the dental porcelain is then overlaid onto the foil which supports the porcelain during the subsequent firing cycles necessary. This procedure is extremely technique-sensitive, requiring skillful and meticulous processing. Proper and accurate shaping of the platinum foil over the replica is extremely difficult and frequently leads to poorly fitted crowns. Additional problems of fit arise due to the shrinkage of the dental porcelain which often distorts the thin platinum matrix during firing. It is therefore more difficult to obtain the desired fit with this crown as compared to the cast metal porcelain composite.